In the prior art of audio signal transmission, it is well known to carry out, before transmission, an operation of encoding an original signal. As for the received signal, this undergoes a reverse decoding operation. This encoding can be a bit rate reduction encoding. Known bit rate reduction encoders are for example transform type encoders such as the MPEG1, MPEG2 or MPEG4-GA encoders, CELP type encoders and even parametric type encoders, such as a parametric MPEG4 type encoder.
In bit rate reduction audio encoding, the audio signal must often undergo passband limiting when the bit rate becomes low. This passband limiting is necessary in order to avoid the introduction of audible quantization noise in the encoded signal. It is then desirable to complete the spectral content of the original signal as far as possible.
Band widening is known in the prior art, such as for example the spectral widening method known by the name HFR (High-Frequency Regeneration) method. The decoded low-frequency signal, with limited band, is subjected to a non-linear device in order to obtain a signal enriched with harmonics. This signal, after whitening and shaping based on information describing the spectral envelope of the full-band signal before encoding, allows the generation of a high-frequency signal corresponding to the high-frequency content of the signal before encoding.
Digital audio encoding systems which use high-frequency spectrum reconstruction techniques at encoder level as well as at decoder level are also known.
These systems perform an adaptation over time of the cut-off frequency between the low-frequency band encoded by an encoder, referred to as the core encoder, and the high-frequency band encoded by an HER system, referred to as a band extension encoder.
In this case, the core encoder and the band extension encoder share the passband according to the adapted cut-off frequency.
This type of system is particularly advantageous for encoding audio signals.
Certain communication networks such as the Internet, wireless communication networks and others do not guarantee a perfect routing of data between the sender and the addressee. Some data may thus never arrive at the addressee or arrive there to late. In arriving too late, the addressee considers them as lost.
In these networks, the passband available for routing the data also continuously varies considerably.
In other networks, such as radio networks, some of the data amongst the transmitted data have a higher priority than others. Highly effective error-correcting codes are associated with these, ensuring correct decoding, and therefore no transmission losses. Others, on the other hand, are less important and lower-performance error-correcting codes, perhaps even none, are associated with them. The latter data are subject to the hazards of the network and decoding might well not be achievable.
In certain encoding systems such as those used in the MPEG4 standard, it may be, following transmission errors, that the signal of a certain frequency band of the spectrum of the encoded signal can no longer be decoded, these frequency components then being lost.
Thus, even if the encoding of the audio signal has been performed in the best possible manner, the decoding of signals transmitted on such networks comprises a number of faults related to these networks.